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Full text of "Treatise On Applied Analytical Chemistry(Vol-1)"

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POTABLE WATERS

Samples should be kept in a cold, dark place and be analysed as soon
as possible.

Physical Characters.—The colour,  clearness,  odour and taste are

noted.

When the sample is taken, its temperature and that of the surrounding
air are observed. The reaction towards litmus is also tested.

If the water is turbid, it is left at rest until clear and then filtered through
a dry filter, the analysis being carried out on the filtrate. If the insoluble
residue is appreciable in amount, it is weighed on a tared filter and its
nature investigated.

1.   Partial Analysis

The partial analysis of a potable water includes, besides observation
of the above physical characters, the following estimations.

1.  Fixed Residue.—In a platinum dish or crucible 200 c.c., or more
if the proportion of mineral matter is low, of the water are evaporated to
dryness on a water-bath or air-bath.   The residue is dried in an oven,
first at 100-105° "to constant weight, and then at 180° to constant weight.

• The residue dried at 180° is then heated to dull redness over a naked
flame to see if it blackens or emits an odour of burning organic matter.
When marked blackening is observed, the calcination is continued t.ntil
all the carbon is burnt, the residue being then moistened with ammonium
carbonate solution, calcined again at a dull red heat and rcweighed. The
difference in weight between the residue dried at 180° and that heated to
dull redness gives approximately the quantity of non-volatile organic matter
in the water.

As a rule, the residue dried at 180° is expressed in grams per 100 litres
of the water.

2.  Hadrness.—A water is said to be hard when it dissolves soap badly
and does not cook vegetables well, these being properties dependent essen-
tially on the calcium and magnesium salts contained in the water;  the
degree of hardness thus represents the whole of the calcium and magnesium
salts calculated as either calcium carbonate or oxide.    Total hardness ia
that due to all the calcium and magnesium salts dissolved in the water;
-permanent hardness, that due to such of these salts as remain in solution
after the water is boiled ; temporary hardness, that due to those salts which
are precipitated from the water on boiling, that is, which were originally
dissolved as bicarbonates.

The two following methods are commonly employed for the estimation
of hardness:

(a] BOUTRON AND BoUDET'S METHOD.

This requires:

(1)  A solution of pure calcium chloride containing 0-25 gram CaCl8
per litre.1

(2)  Soap solution, prepared by dissolving in the hot 50 grams of white

1 This may be prepared by dissolving 0-2253 gram of pure calcium carbonate in
hydrochloric acid and evaporating repeatedly to dryness to expel the excess of acicl.........'Sl)J>